RCPs Historical
(a)
1990 2000 2010 2020 2030 2040 2050
−0.5
1990 2000 2010 2020 2030 2040 2050
−0.5
2.5 Indicative likely range for annual means ALL RCPs (5−95% range, two reference periods) ALL RCPs min−max (299 ensemble members) Observational uncertainty (HadCRUT4)
Figure TS.14 | Synthesis of near-term projections of global mean surface air temperature (GMST). (a) Projections of annual mean GMST 1986–2050 (anomalies relative to 1986–2005) under all RCPs from CMIP5 models (grey and coloured lines, one ensemble member per model), with four observational estimates (Hadley Centre/Climatic Research Unit gridded surface temperature data set 4 (HadCRUT4), European Centre for Medium Range Weather Forecasts (ECMWF) interim re-analysis of the global atmosphere and surface conditions (ERA-Interim), Goddard Institute for Space Studies Surface Temperature Analysis (GISTEMP), National Oceanic and Atmospheric Administration (NOAA)) for the period 1986–2012 (black lines). (b) As (a) but showing the 5 to 95% range of annual mean CMIP5 projections (using one ensemble member per model) for all RCPs using a reference period of 1986–2005 (light grey shade) and all RCPs using a reference period of 2006–2012, together with the observed anomaly for (2006–2012) minus (1986–2005) of 0.16°C (dark grey shade). The percentiles for 2006 onwards have been smoothed with a 5-year running mean for clarity. The maximum and minimum values from CMIP5 using all ensemble members and the 1986–2005 reference period are shown by the grey lines (also smoothed). Black lines show annual mean observational estimates. The red shaded region shows the indicative likely range for annual mean GMST during the period 2016–2035 based on the ‘ALL RCPs Assessed’ likely range for the 20-year mean GMST anomaly for 2016–2035, which is shown as a black bar in both (b) and (c) (see text for details). The temperature scale relative to 1850-1900 mean climate on the right-hand side assumes a warming of GMST prior to 1986–2005 of 0.61°C estimated from HadCRUT4. (c) A synthesis of projections for the mean GMST anomaly for 2016–2035 relative to 1986–2005. The box and whiskers represent the 66% and 90% ranges. Shown are: unconstrained SRES CMIP3 and RCP CMIP5 projections; observationally constrained projections for the SRES A1B and, the RCP4.5 and 8.5 scenarios; unconstrained projections for all four RCP scenarios using two reference periods as in (b) (light grey and dark grey shades), consistent with (b); 90%
range estimated using CMIP5 trends for the period 2012–2035 and the observed GMST anomaly for 2012; an overall likely (>66%) assessed range for all RCP scenarios. The dots for the CMIP5 estimates show the maximum and minimum values using all ensemble members. The medians (or maximum likelihood estimate; green filled bar) are indicated by a grey band. (Adapted from Figure 11.25.) See Section 11.3.6 for details. {Figure 11.25}
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Technical Summary
TS.5.4.3 Projected Near-term Changes in the Water Cycle
Zonal mean precipitation will very likely increase in high and some of the mid latitudes, and will more likely than not decrease in the subtrop-ics. At more regional scales precipitation changes may be dominated by a combination of natural internal variability, volcanic forcing and anthropogenic aerosol effects. {11.3.2}
Over the next few decades increases in near-surface specific humidity are very likely. It is likely that there will be increases in evaporation in many regions. There is low confidence in projected changes in soil moisture and surface runoff. {11.3.2}
In the near term, it is likely that the frequency and intensity of heavy precipitation events will increase over land. These changes are primar-ily driven by increases in atmospheric water vapour content, but also affected by changes in atmospheric circulation. The impact of anthro-pogenic forcing at regional scales is less obvious, as regional-scale changes are strongly affected by natural variability and also depend on the course of future aerosol emissions, volcanic forcing and land use changes (see also TFE.9). {11.3.2}
TS.5.4.4 Projected Near-term Changes in Atmospheric Circulation Internally generated climate variability and multiple RF agents (e.g., volcanoes, GHGs, ozone and anthropogenic aerosols) will all contrib-ute to near-term changes in the atmospheric circulation. For example, it is likely that the annual mean Hadley Circulation and the SH mid-lat-itude westerlies will shift poleward, while it is likely that the projected recovery of stratospheric ozone and increases in GHG concentrations will have counteracting impacts on the width of the Hadley Circula-tion and the meridional posiCircula-tion of the SH storm track. Therefore it is unlikely that they will continue to expand poleward as rapidly as in recent decades. {11.3.2}
There is low confidence in near-term projections of the position and strength of NH storm tracks. Natural variations are larger than the pro-jected impact of GHGs in the near term. {11.3.2}
There is low confidence in basin-scale projections of changes in inten-sity and frequency of tropical cyclones in all basins to the mid-21st century. This low confidence reflects the small number of studies exploring near-term tropical cyclone activity, the differences across published projections of tropical cyclone activity, and the large role for natural variability. There is low confidence in near-term projections for increased tropical cyclone intensity in the Atlantic; this projection is in part due to projected reductions in aerosol loading. {11.3.2}
TS.5.4.5 Projected Near-term Changes in the Ocean
It is very likely that globally averaged surface and vertically averaged ocean temperatures will increase in the near-term. In the absence of multiple major volcanic eruptions, it is very likely that globally aver-aged surface and depth-averaver-aged temperatures averaver-aged for 2016–
2035 will be warmer than those averaged over 1986–2005. {11.3.3}
It is likely that salinity will increase in the tropical and (especially) sub-tropical Atlantic, and decrease in the western sub-tropical Pacific over the next few decades. Overall, it is likely that there will be some decline in the Atlantic Meridional Overturning Circulation by 2050 (medium confidence). However, the rate and magnitude of weakening is very uncertain and decades when this circulation increases are also to be expected. {11.3.3}
TS.5.4.6 Projected Near-term Changes in the Cryosphere
A nearly ice-free Arctic Ocean (sea ice extent less than 106 km2 for at least five consecutive years) in September is likely before mid-century under RCP8.5 (medium confidence). This assessment is based on a subset of models that most closely reproduce the climatological mean state and 1979 to 2012 trend of Arctic sea ice cover. It is very likely that there will be further shrinking and thinning of Arctic sea ice cover, and decreases of northern high-latitude spring time snow cover and near surface permafrost as GMST rises (Figures TS.17 and TS.18). There is low confidence in projected near-term decreases in the Antarctic sea ice extent and volume. {11.3.4}
TS.5.4.7 Possibility of Near-term Abrupt Changes in Climate There are various mechanisms that could lead to changes in global or regional climate that are abrupt by comparison with rates experienced in recent decades. The likelihood of such changes is generally lower for the near term than for the long term. For this reason the relevant mechanisms are primarily assessed in the TS.5 sections on long-term changes and in TFE.5. {11.3.4}
TS.5.4.8 Projected Near-term Changes in Air Quality
The range in projections of air quality (O3 and PM2.5 in surface air) is driven primarily by emissions (including CH4), rather than by physi-cal climate change (medium confidence). The response of air qual-ity to climate-driven changes is more uncertain than the response to emission-driven changes (high confidence). Globally, warming decreases background surface O3 (high confidence). High CH4 levels (such as RCP8.5 and SRES A2) can offset this decrease, raising 2100 background surface O3 on average by about 8 ppb (25% of current levels) relative to scenarios with small CH4 changes (such as RCP4.5 and RCP6.0) (high confidence). On a continental scale, projected air pollution levels are lower under the new RCP scenarios than under the SRES scenarios because the SRES did not incorporate air quality legis-lation (high confidence). {11.3.5, 11.3.5.2; Figures 11.22 and 11.23ab, AII.4.2, AII.7.1–AII.7.4}
Observational and modelling evidence indicates that, all else being equal, locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of O3 and PM2.5 (medium confidence). Local emis-sions combined with background levels and with meteorological con-ditions conducive to the formation and accumulation of pollution are known to produce extreme pollution episodes on local and regional scales. There is low confidence in projecting changes in meteorologi-cal blocking associated with these extreme episodes. For PM2.5, cli-mate change may alter natural aerosol sources (wildfires, wind-lofted
TS dust, biogenic precursors) as well as precipitation scavenging, but no
confidence level is attached to the overall impact of climate change on PM2.5 distributions. {11.3.5, 11.3.5.2; Box 14.2}
TS.5.5 Long-term Climate Change
TS.5.5.1 Projected Long-term Changes in Global Temperature Global mean temperatures will continue to rise over the 21st century under all of the RCPs. From around the mid-21st century, the rate of global warming begins to be more strongly dependent on the scenario (Figure TS.15). {12.4.1}
Under the assumptions of the concentration-driven RCPs, GMSTs for 2081–2100, relative to 1986–2005 will likely be in the 5 to 95% range of the CMIP5 models; 0.3°C to 1.7°C (RCP2.6), 1.1 to 2.6°C (RCP4.5), 1.4°C to 3.1°C (RCP6.0), 2.6°C to 4.8°C (RCP8.5) (see Table TS.1). With high confidence, the 5 to 95% range of CMIP5 is assessed as likely rather than very likely based on the assessment of TCR (see TFE.6).
42 models
3925 4232
1217 12
Figure TS.15 | (Top left) Total global mean radiative forcing for the four RCP scenarios based on the Model for the Assessment of Greenhouse-gas Induced Climate Change (MAGICC) energy balance model. Note that the actual forcing simulated by the CMIP5 models differs slightly between models. (Bottom left) Time series of global annual mean surface air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi-model mean (solid lines) and ±1.64 standard deviation (5 to 95%) across the distribution of individual models (shading), based on annual means. The 1.64 standard deviation range based on the 20 yr averages 2081–2100, relative to 1986–2005, are interpreted as likely changes for the end of the 21st century. Discontinuities at 2100 are due to different numbers of models performing the extension runs beyond the 21st century and have no physical meaning. Numbers in the same colours as the lines indicate the number of different models contribut-ing to the different time periods. Maps: Multi-model ensemble average of annual mean surface air temperature change (compared to 1986–2005 base period) for 2016–2035 and 2081–2100, for RCP2.6, 4.5, 6.0 and 8.5. Hatching indicates regions where the multi-model mean signal is less than one standard deviation of internal variability. Stippling indicates regions where the multi-model mean signal is greater than two standard deviations of internal variability and where 90% of the models agree on the sign of change. The number of CMIP5 models used is indicated in the upper right corner of each panel. Further detail regarding the related Figures SPM.7a and SPM.8.a is given in the TS Supplementary Material. {Box 12.1; Figures 12.4, 12.5, 12.11; Annex I}
The 5 to 95% range of CMIP5 for global mean temperature change is also assessed as likely for mid-21st century, but only with medium confidence. With respect to 1850–1900 mean conditions, global temperatures averaged in the period 2081–2100 are projected to likely exceed 1.5°C above 1850–1900 values for RCP4.5, RCP6.0 and RCP8.5 (high confidence) and are likely to exceed 2°C above 1850–1900 values for RCP6.0 and RCP8.5 (high confidence). Temperature change above 2°C relative to 1850–1900 under RCP2.6 is unlikely (medium confidence). Warming above 4°C by 2081–2100 is unlikely in all RCPs (high confidence) except for RCP8.5, where it is about as likely as not (medium confidence). {12.4.1; Tables 12.2, 12.3}
TS.5.5.2 Projected Long-term Changes in Regional Temperature There is very high confidence that globally averaged changes over land will exceed changes over the ocean at the end of the 21st century by a factor that is likely in the range 1.4 to 1.7. In the absence of a strong reduction in the Atlantic Meridional Overturning, the Arctic region is projected to warm most (very high confidence) (Figure TS.15). As
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Technical Summary
GMST rises, the pattern of atmospheric zonal mean temperatures show warming throughout the troposphere and cooling in the stratosphere, consistent with previous assessments. The consistency is especially clear in the tropical upper troposphere and the northern high latitudes.
{12.4.3; Box 5.1}
It is virtually certain that, in most places, there will be more hot and fewer cold temperature extremes as global mean temperatures increase. These changes are expected for events defined as extremes on both daily and seasonal time scales. Increases in the frequency, duration and magnitude of hot extremes along with heat stress are expected; however, occasional cold winter extremes will continue to occur. Twenty-year return values of low-temperature events are pro-jected to increase at a rate greater than winter mean temperatures in most regions, with the largest changes in the return values of low temperatures at high latitudes. Twenty-year return values for high-temperature events are projected to increase at a rate similar to or greater than the rate of increase of summer mean temperatures in most regions. Under RCP8.5 it is likely that, in most land regions, a cur-rent 20-year high-temperature event will occur more frequently by the end of the 21st century (at least doubling its frequency, but in many regions becoming an annual or 2-year event) and a current 20-year low-temperature event will become exceedingly rare (See also TFE.9).
{12.4.3}
Models simulate a decrease in cloud amount in the future over most of the tropics and mid-latitudes, due mostly to reductions in low clouds.
Changes in marine boundary layer clouds are most uncertain. Increases in cloud fraction and cloud optical depth and therefore cloud reflection are simulated in high latitudes, poleward of 50°. {12.4.3}
TS.5.5.3 Projected Long-term Changes in Atmospheric Circulation Mean sea level pressure is projected to decrease in high latitudes and increase in the mid-latitudes as global temperatures rise. In the trop-ics, the Hadley and Walker Circulations are likely to slow down. Pole-ward shifts in the mid-latitude jets of about 1 to 2 degrees latitude are likely at the end of the 21st century under RCP8.5 in both hemi-spheres (medium confidence), with weaker shifts in the NH. In austral summer, the additional influence of stratospheric ozone recovery in the SH opposes changes due to GHGs there, though the net response varies strongly across models and scenarios. Substantial uncertainty and thus low confidence remains in projecting changes in NH storm tracks, especially for the North Atlantic basin. The Hadley Cell is likely to widen, which translates to broader tropical regions and a pole-ward encroachment of subtropical dry zones. In the stratosphere, the Brewer–Dobson circulation is likely to strengthen. {12.4.4}
2046–2065 2081–2100
Scenario Mean Likely rangec Mean Likely rangec
Global Mean Surface
Scenario Mean Likely ranged Mean Likely ranged
Global Mean Sea Level Rise (m)b
RCP2.6 0.24 0.17 to 0.32 0.40 0.26 to 0.55
RCP4.5 0.26 0.19 to 0.33 0.47 0.32 to 0.63
RCP6.0 0.25 0.18 to 0.32 0.48 0.33 to 0.63
RCP8.5 0.30 0.22 to 0.38 0.63 0.45 to 0.82
Notes:
a Based on the CMIP5 ensemble; anomalies calculated with respect to 1986–2005. Using HadCRUT4 and its uncertainty estimate (5−95% confidence interval), the observed warming to the reference period 1986−2005 is 0.61 [0.55 to 0.67] °C from 1850−1900, and 0.11 [0.09 to 0.13] °C from 1980−1999, the reference period for projections used in AR4. Likely ranges have not been assessed here with respect to earlier reference periods because methods are not generally available in the literature for combining the uncertainties in models and observations. Adding projected and observed changes does not account for potential effects of model biases compared to observations, and for natural internal variability during the observational reference period. {2.4; 11.2;
Tables 12.2 and 12.3}
b Based on 21 CMIP5 models; anomalies calculated with respect to 1986–2005. Where CMIP5 results were not available for a particular AOGCM and scenario, they were estimated as explained in Chapter 13, Table 13.5. The contributions from ice sheet rapid dynamical change and anthropogenic land water storage are treated as having uniform probability distributions, and as largely independent of scenario. This treatment does not imply that the contributions concerned will not depend on the scenario followed, only that the current state of knowledge does not permit a quantitative assessment of the dependence. Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. There is medium confidence that this additional contribution would not exceed several tenths of a metre of sea level rise during the 21st century.
c Calculated from projections as 5−95% model ranges. These ranges are then assessed to be likely ranges after accounting for additional uncertainties or different levels of confidence in models.
For projections of global mean surface temperature change in 2046−2065 confidence is medium, because the relative importance of natural internal variability, and uncertainty in non-greenhouse gas forcing and response, are larger than for 2081−2100. The likely ranges for 2046−2065 do not take into account the possible influence of factors that lead to the assessed range for near-term (2016−2035) global mean surface temperature change that is lower than the 5−95% model range, because the influence of these factors on longer term projections has not been quantified due to insufficient scientific understanding. {11.3}
d Calculated from projections as 5−95% model ranges. These ranges are then assessed to be likely ranges after accounting for additional uncertainties or different levels of confidence in models.
For projections of global mean sea level rise confidence is medium for both time horizons.
Table TS.1 | Projected change in global mean surface air temperature and global mean sea level rise for the mid- and late 21st century relative to the reference period of 1986–2005. {12.4.1; Tables 12.2,13.5}
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TS.5.5.4 Projected Long-term Changes in the Water Cycle
On the planetary scale, relative humidity is projected to remain roughly constant, but specific humidity to increase in a warming climate. The projected differential warming of land and ocean promotes changes in atmospheric moistening that lead to small decreases in near-surface relative humidity over most land areas with the notable exception of parts of tropical Africa (medium confidence) (see TFE.1, Figure 1).
{12.4.5}
It is virtually certain that, in the long term, global precipitation will increase with increased GMST. Global mean precipitation will increase at a rate per °C smaller than that of atmospheric water vapour. It will likely increase by 1 to 3% °C–1 for scenarios other than RCP2.6. For RCP2.6 the range of sensitivities in the CMIP5 models is 0.5 to 4% °C–1 at the end of the 21st century. {7.6.2, 7.6.3, 12.4.1}
Changes in average precipitation in a warmer world will exhibit sub-stantial spatial variation under RCP8.5. Some regions will experience increases, other regions will experience decreases and yet others will not experience significant changes at all (see Figure TS.16). There is high confidence that the contrast of annual mean precipitation between dry and wet regions and that the contrast between wet and dry seasons will increase over most of the globe as temperatures increase. The general pattern of change indicates that high latitudes are very likely to experience greater amounts of precipitation due to the increased specific humidity of the warmer troposphere as well as increased transport of water vapour from the tropics by the end of this
Figure TS.16 | Maps of multi-model results for the scenarios RCP2.6, RCP4.5, RCP6.0 and RCP8.5 in 2081–2100 of average percent change in mean precipitation. Changes are shown relative to 1986–2005. The number of CMIP5 models to calculate the multi-model mean is indicated in the upper right corner of each panel. Hatching indicates regions where the multi- model mean signal is less than 1 standard deviation of internal variability. Stippling indicates regions where the multi- model mean signal is greater than 2 standard deviations of internal variability and where 90% of models agree on the sign of change (see Box 12.1). Further detail regarding the related Figure SPM.8b is given in the TS Supplementary Material. {Figure 12.22; Annex I}
century under the RCP8.5 scenario. Many mid-latitude and subtropical arid and semi-arid regions will likely experience less precipitation and many moist mid-latitude regions will likely experience more precipita-tion by the end of this century under the RCP8.5 scenario. Maps of precipitation change for the four RCP scenarios are shown in Figure TS.16. {12.4.2, 12.4.5}
Globally, for short-duration precipitation events, a shift to more intense
Globally, for short-duration precipitation events, a shift to more intense